We intend to perform simultaneous voltage clamp and fluorescence measurements on single living lizard intercostal muscle endplates. Fluorescence will be excited using either a helium cadmium laser, or by a mercury arc lamp. An epilluminator using dichroic mirrors and high numerical aperture water immersion objectives will be used. Fluorescence will be collected via the objective, passed through an adjustable pin hole to select the endplate region, and measured using a photomultiplier. Several classes of specific probes will be used: quinacrine, dansyl acylcholines, rhodamine burgarotoxin, sulfhydryl group irreversible probes. Emission under agonist application (either iontophocitic or in the bath) will be correlated with membrane currents, in order to relate the binding of active drugs to their physiological effect.